Light-emitting diodes (LEDs) are the most efficient among currently available light sources. In particular, white LEDs find a rapidly expanding share in the market as the next-generation light source to replace incandescent lamps, fluorescent lamps, cold cathode fluorescent lamps (CCFL), and halogen lamps. The white LEDs are arrived at by combining a blue LED with a phosphor capable of emission upon blue light excitation. Examples of yellow light-emitting phosphors known to produce pseudo-white light in combination with blue LEDs include Y3Al5O12:Ce, (Y,Gd)3(Al,Ga)5O12:Ce, (Y,Gd)4Al5O22:Ce, Tb3Al5O12:Ce, CaGa2S4:Eu, (Sr,Ca,Ba)2SiO4:Eu, and Ca-α-SiAlON:Eu.
JP-B-3700502 discloses a method for preparing a phosphor by dissolving rare earth elements Y, Gd, and Ce in stoichiometric proportions in an acid, coprecipitating the solution with oxalic acid, firing the coprecipitate to obtain an oxide of the coprecipitate, mixing it with aluminum oxide, and adding ammonium fluoride as flux thereto. The mixture is placed in a crucible and fired in air at 1,400° C. for 3 hours. The fired material is wet milled in a ball mill, washed, separated, dried, and finally sieved.
When a phosphor is synthesized by such a conventional process of particle mixing and solid-phase reaction, phosphor properties are often degraded because some particles may be left unreacted and ball milling can detract from crystallinity and generate fines. The particles have irregular shape and a broad particle size distribution, indicating the presence of phosphor particles of widely varying size. When such phosphor particles are dispersed in a resin which is used for the encapsulation of LEDs, the amount, distribution and particle size of the phosphor in the resin widely vary. This leads to variations in the chromaticity of white LED and causes a reduction of production yield.
Citation List
Patent Document 1: JP-B-3700502